The present invention relates to generally a cathode-ray tube device and more particularly an electromagnetic-deflection type cathode-ray device which comprises a cathode-ray tube and its driving circuitry and which is so designed and constructed that high-quality and high-resolution images can be represented over the whole surface of a phosphor screen of the cathode-ray tube.
In the case of the color cathode-ray tube and especially in the case of the in-line type electromagnetic-deflection cathode-ray tube in which electron guns are arranged in a horizontal line, a saddle or toroidal type deflecting yoke is used so that the horizontal deflection field is distorted in the form of a pincushion while the vertical deflection field is distorted in the form of a barrel and consequently the self-convergence effect can be attained. With the saddle or toroidal type deflecting yoke, the convergence system can be much simplified. However, the spot of the electron beam (which will be referred to as the "beam spot" in this specification) is distorted from a true circle particularly at the portions adjacent to the peripheries of the phosphor screen due to the distortions of the deflection fields so that the resolution is degraded accordingly. The beam spot consists of a bright core portion and a relatively dim haze portion adjacent to the core portion. If the beam current is low and the vertical diameter of the beam spot is reduced too much, sharp moires are produced due to interference between the scanning beam and the apertures of the shadow mask.
Even in the case of an electromagnetic-deflection type monochrome cathode-ray tube, if the deflection fields are not uniform, the resolution is degraded at the portions adjacent to the peripheries of the screen.
The decrease in resolution due to the distortions of the deflection fields can be remedied by reducing the diameter of the electron beam passing through the main lens and the deflecting yoke. However, if spacing between the cathode of an electron gun and an electrostatic lens is reduced or if the prefocusing system is employed to finely focus the electron beam, the magnification of the electrostatic lens is increased excessively so that the beam spot is undesirably and unavoidably increased in diameter at the center of the screen. In order to overcome this problem, there has been devised and demonstrated a system in which the electrostatic field produced by the prefocusing system is made axially asymmetrical so that the magnification in the vertical direction can be greater than the magnification in the horizontal direction. Then, the cross section of the electron beam can be elongated in the horizontal direction so that the distortions in the vertical direction of the beam spot at the portions adjacent to the peripheries of the screen can be remedied. Since the electron beam passing through the deflecting yoke is elongated in cross section, the beam spot which appears at the portion close to the peripheries of the screen can be maintained in the form of a true circle so that moires can be avoided. However, the beam spot is elongated in the vertical direction at the center of the screen and consequently the resolution in the vertical direction is degraded. Thus, the resolution at the peripheries of the screen is improved at the sacrifice of the resolution at the center thereof.